1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method for switching a service type according to QoS (Quality of Service) in a mobile communication system supporting time division multiplexing (TDM) for a shared link.
2. Description of the Related Art
A 3rd generation mobile communication system includes CDMA2000 (Code Division Multiple Access 2000), WCDMA/UMTS (Wideband CDMA/Universal Mobile Telecommunications System), GPRS (General Packet Radio System), and CDMA2000 1xEV-DO (Evolution Data Only) mobile communication systems. Unlike a typical 2nd generation mobile communication system that supports only a voice service or a low-speed data service, the 3rd generation mobile communication system supports the voice service, a high-speed packet data service, and a moving picture communication service. The mobile communication system includes a base station controller (BSC), a base transceiver system (BTS), and a mobile station (MS). The BSC is connected to the BTS by wire, and the BTS is connected to the MS through a radio channel, for communication.
In the mobile communication system, when the BSC communicates with a specified MS among a plurality of MSs through its lower BTS, the BSC sequentially transmits transmission data packets to the BTS to which the specified MS belongs. The BTS then buffers (or stores) the received data packets in its buffer, and sequentially transmits the buffered data packets to the specified MS for a time period where a radio resource is available. A technique for sharing one radio resource, or one transmission channel, by a plurality of MSs is called “time division multiplexing (TDM).”
In a mobile communication system transmitting data packets by the TDM, an operation of determining a time period for which the data packets are transmitted to MS is called “scheduling,” and radio resources of the BTS are properly assigned to a plurality of MSs by the scheduling. A scheduler for the scheduling operation exists in the BTS, and the MS periodically reports the quality of a BTS signal received over a radio channel to the scheduler. The scheduler selects a particular MS, to which it will transmit data, every scheduling period, considering quality information of radio channels received from a plurality of MSs and absence/presence of data to be transmitted to each MS.
Meanwhile, a data service in the conventional mobile communication system is based on the so-called “best effort service.” That is, a condition of a data service provided to a particular mobile subscriber (or MS) is determined simply based on a radio channel quality of the subscriber and availability of a radio resource with which each BTS services subscribers. Therefore, an improved mobile communication system provides differential services according to a QoS (Quality of Service) class, and guarantees a required data rate for an MS subscriber.
A radio channel quality of an MS is constantly varied according to various factors such as movement of the MS and fading. Thus, if a radio environment becomes very poor, it is not possible to support a required data rate for the MS at the start of a service. However, even in this case, the mobile communication system attempts to continuously guarantee the required data rate, thereby wasting radio resources that are assignable to other MSs.